Cationic:anionic surfactant mixtures adsorbed at an oil–water interface stabilize foams in the presence of oil, making them essential to the oil, gas, and firefighting industries. The oil tolerance of foams stabilized by surfactant mixtures, relative to pure (unmixed) cationic and anionic surfactants, results from the mixtures’ enhanced flexibility in tailoring the physicochemical properties of the interface. To judiciously employ these mixtures, it is necessary to characterize the structure–function property relationship of their surfactant monolayers that lend to oil-tolerant/intolerant foams. In this work, we employ interfacial tensiometry and vibrational sum frequency spectroscopy to determine the composition (surfactant population and cationic:anionic ratio) and the structure (surfactant alkyl tail conformation) of monolayers prepared at the oil–water interface by 1:1 DTAB:SDS (dodecyltrimethylammonium bromide:sodium dodecyl sulfate) mixtures. We show that the interfacial surfactant density of 1:1 DTAB:SDS mixtures greatly exceeds that of pure DTAB and SDS at similar concentrations up to and beyond their respective critical micelle concentration. The enhanced interfacial adsorption of these mixtures is due to the adsorption of stoichiometric 1:1 DTAB:SDS surfactant pairs that form through the attractive electrostatic interactions between surfactant headgroups. We find that these paired surfactants preferentially adsorb at the interface, causing the interfacial DTAB:SDS ratio to be nearly 1:1. Additionally, we find that the SDS tail is more conformationally ordered than the DTAB tail, even though they are expected to be conformationally identical along the entire tail, since they are likely conjoined through van der Waals interactions. This leads to the conclusion that the surfactant pairs are in a staggered arrangement at the interface. These findings help to uncover molecular factors that contribute to the enhanced oil tolerance, and in some cases oil intolerance, of foams stabilized by cationic:anionic surfactant mixtures.

中文翻译：

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两条尾巴的故事：化学计量 1：1 DTAB：吸附在油-水界面的 SDS 对的尾部排序


吸附在油-水界面处的阳离子：阴离子表面活性剂混合物可在油存在下稳定泡沫，使其对石油、天然气和消防行业至关重要。相对于纯（未混合）阳离子和阴离子表面活性剂，表面活性剂混合物稳定的泡沫的耐油性是由于混合物在调整界面的物理化学性质方面具有更强的柔韧性。为了明智地使用这些混合物，有必要表征其表面活性剂单层的结构-功能特性关系，这些单层可用于耐油/不耐受泡沫。在这项工作中，我们采用界面张力测定法和振动和频率谱来确定由 1：1 DTAB：SDS（十二烷基三甲基溴化铵：十二烷基硫酸钠）混合物在油水界面制备的单层的组成（表面活性剂数量和阳离子：阴离子比）和结构（表面活性剂烷基尾部构象）。我们表明，1：1 DTAB：SDS 混合物的界面表面活性剂密度大大超过相似浓度的纯 DTAB 和 SDS 的密度，达到并超过它们各自的临界胶束浓度。这些混合物的界面吸附增强是由于化学计量 1：1 DTAB：SDS 表面活性剂对的吸附，这些表面活性剂对是通过表面活性剂头基之间有吸引力的静电相互作用形成的。我们发现这些成对的表面活性剂优先吸附在界面处，导致界面 DTAB：SDS 比率接近 1：1。此外，我们发现 SDS 尾部比 DTAB 尾部更有序，即使预计它们在整个尾部的构象相同，因为它们可能是通过范德华相互作用结合的。 这由此得出结论，表面活性剂对在界面处交错排列。这些发现有助于揭示导致阳离子：阴离子表面活性剂混合物稳定的泡沫增强耐油性（在某些情况下导致耐油性）的分子因素。